True or False: All fluids are suitable for use in a hydraulic system.

• A. True
• B. False
• C. Not recommended
• D. Only synthetic

Answer: (B)

In a hydraulic system, the fluid must fulfill multiple demanding roles: transmit pressure, lubricate moving parts, seal against leaks, resist wear, and carry away heat. Because of these duties, only fluids that match the system’s materials, seals, and operating conditions will work properly. Many substances won’t meet these requirements: some can swell or degrade elastomer seals or corrosion-control coatings, others aren’t compatible with the metals used, and some have viscosities that are either too high or too low for the pump and valve clearances at operating temperatures. Fire safety considerations in certain systems also limit choices to fluids designed to resist ignition. So, not all fluids are suitable; only those specifically designed and specified for a given hydraulic system will perform reliably.